Reactive hot melt adhesives ("reactive hot melts") are comprised of isocyanate-terminated polyurethane polymers, often referred to as "prepolymers", that react with surface or ambient moisture in order to chain-extend and form a second polyurethane polymer. Reactive hot melt adhesives are also known as "polyurethane hot melt adhesives".
Unlike conventional hot melt adhesives, which can be heated to a liquid state and cooled to a solid state repeatedly, a reactive hot melt undergoes an irreversible chemical reaction to a solid "cured" form once dispensed in the presence of ambient moisture. Reactive hot melts are therefore useful for applications which are exposed to high temperatures; an article adhered with a conventional hot melt would become unglued if exposed to high temperatures which would cause the conventional hot melt adhesive to melt to a liquid state. Articles for which reactive hot melts are particularly useful as adhesives include architectural components on building exteriors and components of recreational vehicles, such as cars and vans.
Reactive hot melts will bond to a variety of substrates including plastics, woods, fabrics and metals, making them ideal candidates for bonding dissimilar substrates. They are flexible and durable by nature, and may be used in extreme temperature ranges of -30 degrees up to about +150 degrees Celsius, while providing excellent moisture and chemical resistance.
With a reactive hot melt adhesive, the bond between two substrates is initially made by the strength of the solid form of the adhesive after it has cooled from a liquid form, which typically occurs after a period of seconds after the adhesive is dispensed in liquid form. This initial bond holds the two substrates together until the solid adhesive cures to its most durable, irreversibly-solid form, which takes place in the presence of moisture, typically after a period of hours, or even days.
Certain applications benefit from fast setting adhesives, that is adhesives which convert quickly from liquid melt to solid state once applied. For example, a high-speed continuous adhesion process often requires that a bond between two sheet-like components (such as between a laminate or veneer and a solid core) be made quickly as the bonded components are extruded from a roller. Thus, various measures have been taken to shorten the setting speed of reactive hot melt adhesives, that is to decrease the amount of time it takes for a reactive hot melt to convert from liquid to solid state.
Meckel et al. (U.S. Pat. No. 5,115,073, Date of Patent May 19, 1992), for example, teach polyurethane systems which are prepared by reacting, inter alia, polyester diols, such as dodecane diol, which have a melting point of 65 degrees Celsius to 150 degrees Celsius (preferably from 70 degrees to 145 degrees Celsius). Systems prepared from these relatively high melting point polyester diols taught by Meckel et al. crystallize at the polyester diol melting point and thus set quicker than conventional reactive hot melt adhesives.
Muller et al. (U.S. Pat. No. 5,019,638, Date of Patent May 28, 1991) disclose a broad range of aliphatic hydroxypolyesters for preparing rapidly setting reactive hot melt adhesives. Muller et al. describe the hydroxypolyesters as being prepared from dicarboxylic acids with 8, 10 or 12 methylene groups and diols with 6 to 12 methylene groups. Muller et al., do not disclose the significance of using a low hydroxyl number composition of polyester diols to achieve fast setting reactive hot melt adhesives.